Link adaptation in wireless networks for throughput maximization under retransmissions

ABSTRACT

The present invention implements a method and system for dynamically adapting the modulation and coding scheme for radio links in a wireless communications network based on a retransmission environment model in order to maximize throughput and most efficiently allocate bandwidth resources. The present invention encompasses a refined calculus and methodology for deriving the link adaptation thresholds in a retransmission environment using a complex model and analysis of the retransmission environment. The present invention holds particular application for wireless data communications as opposed to real time data services because it is based on a retransmission model applicable primarily for data services. A critical component of this new link adaptation system is a “no transmission” cutoff mode that is selected for SIR below a base threshold value. This new mode prevents system instability and misallocation of bandwidth in a wireless communication system.

PRIOR PROVISIONAL PATENT APPLICATION

The present application claims the benefit under 35 U.S.C. § 119(e) ofU.S. Provisional Application No. 60/096,006 filed Aug. 10, 1998.

FIELD OF THE INVENTION

The present invention relates generally to wireless communicationssystems. In particular, the invention concerns a dynamic link adaptationprocess that offers increased throughput and bandwidth allocationefficiency with particular benefit for wireless data services.

BACKGROUND INFORMATION

Interest in wireless data communications has grown rapidly in the pastfew years due to the growth of the Internet. The nature of the datacarried over a wireless network is highly determinative of the type ofarchitecture required for efficient and reliable communications. The keyto meeting the increasing demand for wireless services is thedevelopment of high performance radio systems that take the uniquefeatures of the data traffic into account. For example, the architectureof a real time communications system carrying voice and/or videodiverges greatly from the design considerations for data communicationssystems. Compared to voice and other real-time traffic, data trafficusually has a minimum tolerance for transmission errors and a hightolerance of transmission delay. As a result, packet retransmissions arepossible and often necessary. For data applications, the techniques ofpacket switching, dynamic resource assignment and link adaptation aremore suitable than conventional techniques such as circuit switching,fixed resource allocation and fixed transmission schemes.

The need for re-engineering of communications systems in order toaccommodate the needs of data traffic has been recognized. For example,several existing systems, such as Cellular Digital Packet Data (CDPD),Global System for Mobile Communication (GSM) and IS-136 have thecapacity to support data services. However, these systems employ circuitswitching (except for CDPD) and offer only low data rates. Currently,the data rate of GSM ranges from 2.4 kbps to 9.6 kbps. To enhance thedata capability of GSM, a new service called the General Packet RadioService (GPRS) has been proposed. In addition, the EuropeanTelecommunications Standards Institute (ETSI) has standardized aspecification entitled Enhanced Data Rates for GSM Evolution (EDGE) asan attractive GSM evolution for providing broadband data services. BothEDGE and IS-136 utilize link adaptation in order to maximize throughputand promote bandwidth efficiency.

Link adaptation is a continuous process in which the attributes of eachlink within a communications system are dynamically updated to maximizethroughput (or some other parameter) and efficiently utilize theavailable bandwidth according to a set of criteria. Typically, a linkadaptation scheme consists of a set of modes each incorporating adifferent modulation/coding scheme or some other link parametercontrolling the data rate. Each mode and corresponding modulation/codingscheme has an associated set of performance attributes. For example, theblock error rate (BLER) is an important parameter in a link adaptationsystem. BLER is the probability that a block of bits transmitted fromthe receiver to the transmitter contains an error after decoding. BLERis a function of the signal-to-interference ratio (SIR) (ratio of signalto interference power) at the receiver such that each mode has acharacteristic BLER curve as a function of SIR. Each modulation/codingscheme is also associated with a radio interface rate R, which is theactual rate of information transmission after accounting for the codingoverhead. Using the performance attributes BLER and R for each mode, thethroughput (measure of the actual bit transmission rate from transmitterto receiver) for each mode can be described as a function of SIR.

Link adaptation is accomplished by establishing a set of thresholdvalues for choosing different transmission modes. These threshold valuesare used to determine the selection of each mode in the adaptationscheme based on some real time performance measure such as SIR. A linkadaptation system operates by periodically taking a real timeperformance measure for each link (e.g., SIR at the receiver), comparingthis performance measure with the threshold values for the modes andthen selecting the appropriate mode that will maximize throughput.

The appropriate link adaptation threshold scheme is crucial to realizeperformance gains. If the thresholds are too aggressive, i.e., userswith poor link quality select higher level modulation/coding schemes,the overall system performance will suffer due to excessiveretransmissions. On the other hand, if these thresholds are tooconservative, the system performance will also suffer due to lowspectrum efficiency, which results in prolonged resource occupancy.

Traditional link adaptation schemes use BLER as a basis for establishingthe set of adaptation thresholds. Usually the modulation/coding schemeis updated to maintain the BLER at a particular level, (e.g. 10%). Thus,the BLER establishes an acceptable level of error for a communicationschannel, which is appropriate for real time traffic. On the other hand,data services allow the retransmission of blocks in error at the cost ofdelay. Therefore, BLER is generally not the only criterion for dataservices since the ultimate measure for data services is throughput. Thethroughput depends upon BLER, the transmission rate and the possibilityof retransmissions.

Link adaptation systems for data services typically rely upon throughputcriteria to select the appropriate adaptation mode. For example, thecentral technology of EDGE is a link adaptation scheme that dynamicallyadapts the modulation/coding scheme according to the current linkquality to maximize system throughput. EDGE incorporates two differentmodulation schemes, Offset Quadrature Phase Shift Keying (OQPSK) andOffset 16 Quadrature Amplitude Modulation (O16QAM). Combining these twodifferent modulation schemes with four different coding schemes, EDGEsupports a total of eight possible modulation/coding modes.

The set of thresholds comprising a link adaptation system is derivedfrom a mathematical model of the wireless environment and theperformance attributes for each modulation/coding mode. The choice of anappropriate wireless environment model is critical for establishing thecorrect link adaptation thresholds. For example, conventional linkadaptation schemes such as EDGE are based on a model of ano-retransmission environment that assumes erroneous packets arediscarded and do not increase the load in a system (i.e., packets arenot retransmitted if lost or damaged in the transmission process).

However, retransmissions are in fact necessary for wireless dataservices and the behavior of a retransmission environment divergessignificantly from a no-retransmission environment. In particular, aretransmission environment produces highly complex feedback behaviorthat can result in system instability and degraded performance. Failureto model this complex behavior and derive a correct set of linkadaptation thresholds is a major shortcoming of traditional linkadaptation schemes and can result in significantly degraded systemperformance and instability in the retransmission environment.

For example, retransmissions necessarily increase the load on thesystem, increase interference and lower the SIR. The lowering of the SIRwill result in even more retransmissions until either the system reachesa steady state condition if it exists or the system becomes unstable.Thus, neglecting retransmissions significantly underestimates theinterference in a wireless communications system and link adaptationschemes designed without considering retransmissions will performpoorly.

SUMMARY OF THE INVENTION

The present invention implements a method and system for dynamicallyadapting the modulation and coding scheme for radio links in a wirelesscommunications network based on a retransmission environment model inorder to maximize throughput and most efficiently allocate bandwidthresources. The present invention encompasses a refined calculus andmethodology for deriving the link adaptation thresholds in aretransmission environment using a complex model and analysis of theretransmission environment. The present invention holds particularapplication for wireless data communications as opposed to real timedata services because it is based on a retransmission model applicableprimarily for data services. A critical component of this new linkadaptation system is a “no transmission” cutoff mode that is selectedfor SIR below a base threshold value. This new mode prevents systeminstability and misallocation of bandwidth in a wireless communicationsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the large-scale architecture of a wireless communicationssystem according to one embodiment of the present invention.

FIG. 2 depicts the architecture of a link adaptation system consistingof a set N of modulation/coding schemes according to one embodiment ofthe present invention.

FIG. 3 depicts BLER as a function of SIR for the eight transmissionmodes in EDGE.

FIG. 4 depicts a set of curves relating the SIR₀ for the offered trafficto the throughput for each mode using the EDGE modulation/codingarchitecture according to one embodiment of the present invention.

FIG. 5 depicts an example of the determination of a SIR margin for EDGEmode ECS-5 according to one embodiment of the present invention.

FIG. 6 depicts the derived throughput characteristics for the EDGE modesusing an infinite retransmission model.

FIG. 7 depicts the operation of a no-transmission mode (mode 0)according to one embodiment of the present invention.

FIG. 8 is a flowchart that depicts a set of steps that may beimplemented at a wireless transmitter to utilize a no-transmission modeand perform link adaptation according to one embodiment of the presentinvention.

DETAILED DESCRIPTION

The present invention is based upon an analysis of link adaptationwithin a retransmission environment. The present invention departssignificantly from traditional link adaptation schemes for wireless dataservices, which neglect retransmissions and ultimately produce anerroneous link adaptation framework that produces instability,misallocation of bandwidth and poor system performance.

The retransmission model underlying the present invention generated twocritical discoveries that significantly shaped the present invention.First, the threshold values for a link adaptation system using aretransmission model can be derived from the no-retransmission modelthresholds. The retransmission thresholds are obtained by shifting thethroughput characteristic curves for the no-retransmission model by anamount relating to the difference between the signal to interferenceratio generated by the base offered traffic, SIR₀, and the resultingsignal-to-interference ratio generated due to the base traffic plusretransmissions, SIR.

Second, the retransmission model revealed that there should be notransmission at all below a base threshold SIR (referred to herein asthe “no-transmission” or “mode 0” threshold). Transmitting below this“no-transmission” threshold produces system instability such thatexcessive retransmissions result causing unbounded delay and almost zerothroughput at the receiver. This instability is a product of theretransmission environment itself and is not analyzed or accounted forin conventional link adaptation systems. The complex analysis andinsights underlying the retransmission model are an essentialunderpinning of the present invention and are outlined below. Thisanalysis was summarized from J. Chuang, X. Qiu, “An Improved LinkAdaptation Algorithm and Its Implementation Requirements”, presented atSMG2 EDGE ad hoc on EDGE physical/link layer issues in London, Aug.12-13, 1998, and “Link Adaptation in Wireless Data Networks forThroughput Maximization Under Retransmissions, AT&T TechnicalMemorandum, HA6132000-980714-06TM, July 1998, also submitted to IEEEICC'99, Jun. 6-10, 1999.

In one embodiment of the present invention, the retransmission model wasderived and analyzed using the modulation/coding schemes outlined inEDGE. However, this analysis would apply to any modulation/codingframework. Thus, link adaptation threshold values in a retransmissionenvironment for any modulation/coding architecture can be derived usingthe framework outlined herein. Recently, for example, new modulationschemes were proposed, and the methodology outlined herein can beapplied to them. Furthermore, this same embodiment relied primarily uponan infinite retransmission model, an assumption that packets would beretransmitted until success. However, the basic analysis presentedherein can be used for a retransmission model based upon any arbitrarynumber of retransmissions (e.g., a one retransmission model or a tworetransmissions model).

FIG. 1 depicts the large-scale architecture of a wireless communicationssystem according to one embodiment of the present invention. Transmitter100 communicates with receiver 105 through communications channel 104.Transmitter 100 contains transceiver 115, data module 125,modulator/encoder 110, controller 122 and antenna 140. Controller 122calculates modulation/encoding scheme 150 from quality measure 155 sentfrom receiver 105 and transmits this information to modulator/encoder110. Modulation/encoding scheme 150 is used by modulator/encoder 110 tomodulate and encode data retrieved from data module 125. Themodulated/encoded data is sent to transceiver 115 for transmissionthrough antenna 140 onto communications channel 104. Receiver 105contains decoder 120, controller 122, transceiver 115 and antenna 140.Transceiver 115 is coupled to antenna 140 and communications channel 104from which data is received. Data is sent from transceiver 115 todecoder 120, which is controlled by controller 122. Decoder 120 outputsdecoded data 152 and quality measure 155, which might for example be thecurrent BLER or SIR at the receiver. Quality measure 155 is transmittedback to transmitter 100 through communications channel 104.

FIG. 2 depicts the architecture of a link adaptation system consistingof a set N of modulation/coding schemes 210 according to one embodimentof the present invention. Each scheme nεN (210) is characterized by aset of performance attributes 220 that may include, for example, theradio interference rate R_(n) 225 and BLER_(n) characteristic 227 wherenεN depicts the particular link adaptation mode 210. BLER_(n)characteristic 227 is a function relating the BLER to the SIR at thereceiver 105 for each mode 210. For example, FIG. 3 depicts BLER as afunction of SIR for the eight transmission modes 210 in EDGE. A wirelesstransmission model 240 is associated with the entire link adaptationscheme and is used to derive a throughput characteristic 250 as afunction of SIR for each mode 210. A threshold level 260 is derived foreach mode 210 from the set of throughput characteristics 250 in the linkadaptation system. For each mode 210, the threshold level 260corresponds to the range of SIR over which that mode 210 produces thehighest throughput among all modes 210 in the link adaptation scheme.The set of threshold values 260 dictate the selection of a mode 210based upon real-time measurement of the SIR at the receiver 105.

Analysis of the Infinite Retransmission Model

The wireless environment model 240, which comprises a mathematical andconceptual framework for the wireless transmission environment, is acritical component in determining the set of threshold values 260 foreach mode 210. The throughput characteristic 250 of each mode 210 isderived from wireless environment model 240 and the performanceattributes 220 unique to each mode (i.e. R_(n) and BLER_(n)characteristic where nεN). For example, using a no-retransmissionenvironment model, the throughput S is equal to the probability that ablock is transmitted correctly (I-BLER_(n)) multiplied by the actualdata transmission R_(n)

S _(n) ⁰ =R _(n)(1−BLER _(n)(SIR ₀))  (1)

where SIR₀ is the signal-to-interference ratio for the base offeredtraffic of the system without taking into account any retransmissions.Based upon the no-retransmission environment model 240 as codified inequation (1) and the BLER/SIR relationship depicted in FIG. 3, FIG. 4depicts a set curves relating the SIR₀ for the offered traffic to thethroughput for each mode 210 using the EDGE modulation/codingarchitecture.

However, the curves depicted in FIG. 4 are erroneous in a retransmissionenvironment (such as that required for data services). In fact, usingsuch a link adaptation scheme in a retransmission environment willactually reduce system performance and result in instability in thesystem. For example, in EDGE, in the range of SIR for which ECS-6 ischosen, the average BLER is higher than 65%, meaning that 65% of packetsrequire retransmission. As a result of this BLER, the load in the systemand the interference in the system will be increased substantially. Theincrease of interference will further lower the SIR and cause even moreretransmissions until either the system reaches the steady state if itexists, or the system becomes unstable resulting in a throughput ofzero.

The realization that the traditional no-retransmission model 240 couldnot adequately capture the behavior of the retransmission environmentled to a complex and detailed analysis of an infinite retransmissionenvironment underlying the present invention. To develop a conceptualand mathematical model to account for infinite retransmissions requiredanalysis of the traffic load in a communications system operating in aretransmission environment. ρ₀ represents the average offered traffic inthe communications system neglecting retransmissions. However, theactual load in a transmission system will be higher, represented by ρ,the amount of traffic in the system including base offered traffic andretransmission traffic.

Thus, the total load considering retransmissions ρ will be the offeredload ρ₀ plus the amount of traffic generated by retransmissions. p_(n)represents the probability of using a particular modulation/coding modenεN, where Σ_(nεN) p_(n)=1. For the first retransmission, the additionaltraffic will be the offered traffic ρ₀ multiplied by the probability ofchoosing mode n (nεN) 210 multiplied by the BLER for mode n 210 summedover all modes n (nεN) 210. The same relationship will apply for thesecond retransmission except that the BLER term will be of second orderdue to the two retransmissions. If a user does not change themodulation/coding scheme until the current packet is successfullytransmitted, in the steady state, the load in the transmission systemunder the assumption of infinite retransmissions is given generally by:

$\begin{matrix}{\rho = {\rho_{0} + {\sum\limits_{n \in N}^{\;}\; {\left( {\rho_{0}\rho_{n}} \right) \cdot {BLER}_{n}}} + {\sum\limits_{n \in N}^{\;}\; {\left( {\rho_{0}\rho_{n}} \right) \cdot {BLER}_{n}^{2}}} + \cdots}} & (2)\end{matrix}$

Simplification yields:

$\begin{matrix}{\sum\limits_{n \in N}^{\;}\frac{\rho_{0}p_{n}}{1 - {BLER}_{n}}} & (3)\end{matrix}$

where BLER_(n) is a function of SIR.Using a first order approximation, assuming that the total interferenceI is a linear function of the load ρ, the interference can be describedas:

$\begin{matrix}{I = {\sum\limits_{n \in N}^{\;}\frac{I_{0}p_{n}}{1 - {BLER}_{n}}}} & (4)\end{matrix}$

where I₀ is the interference at the receiver 105 if erroneous packetsare discarded (i.e., no retransmissions). Therefore, in the steadystate, the SIR at a particular link is:

$\begin{matrix}{{SIR} = \frac{{SIR}_{0}}{\sum\limits_{n \in N}^{\;}\frac{p_{n}}{1 - {BLER}_{n}}}} & (5)\end{matrix}$

where SIR₀ is the SIR at a link receiver 105 without consideringretransmissions. Expressed in dB, the SIR at the receiver 105 is:

$\begin{matrix}{{{SIR}{_{dB}{= {SIR}_{0}}}_{dB}} - {10\mspace{14mu} {\log \left( {\sum\limits_{n \in N}^{\;}\frac{p_{n}}{1 - {BLER}_{n}}} \right)}}} & (6)\end{matrix}$

Thus, the SIR at the receiver 105 is the SIR of the offered traffic(i.e. without retransmissions) plus an additional factor C(ρ) (hereinreferred to as the “SIR margin”) corresponding to a reduction in SIR ateach receiver link 105 due to retransmissions

$\begin{matrix}{{{{SIR}{_{dB}{= {SIR}_{0}}}_{dB}} + {C(\rho)}}{{where}\text{:}}} & (7) \\{{C(\rho)} = {{- 10}\mspace{14mu} {\log \left( {\sum\limits_{n \in N}^{\;}\frac{p_{n}}{1 - {BLER}_{n}}} \right)}}} & (8)\end{matrix}$

Relating equation (1) to the preceding analysis, in the steady state,the throughput using the infinite retransmission model 240 is:

S _(n) ^(s) =R _(n)(1−BLER _(n)(SIR ₀ +C(ρ))  (9)

According to one embodiment of the present invention, the determinationof C(ρ) was simplified by making the assumption that all users in thesystem use the same modulation/coding scheme n (nεN) 210, i.e., p_(n)=1,when the SIR margin of mode n (nεN) 210 is considered. Without thisassumption, evaluation of C(ρ) proved to be highly complex since C(ρ) isa function of both {p_(n)} and {BLER_(n)} where {p_(n)) is a function ofthe offered load ρ₀ and many other parameters such as the propagationenvironment. Using this analysis, the determination of the SIR marginwas greatly simplified since C(ρ) is reduced to a function of BLER_(n)alone which itself is a function of SIR (see FIG. 3). Predictions basedupon this assumption have corresponded very closely with measuredexperimental results. Thus, using the assumption that interactionbetween different modes can be decoupled, the SIR at each receiver link105 is:

SIR| _(dB) =SIR ₀|_(dB)+10 log(1−BLER _(n)) where  (10)

C(ρ)=10 log(1−BLER _(n))  (11)

Method for Deriving Threshold Values for Infinite Retransmission Model

Assuming that there is a well defined BLER_(n) characteristic for agiven mode 210 and provided with SIR₀, SIR and C(ρ) can be obtainedanalytically by solving equation (10). According to one embodiment ofthe present invention, the following steps describe a method to evaluateC(ρ):

-   1. For different values of SIR₀, the curves y=SIR₀+C(ρ) and y=SIR    are plotted as a function of SIR.-   2. For a given value of SIR₀, the intersection of the curves    y=SIR₀+C(ρ) and y=SIR yields the SIR that satisfies equation (10).    The SIR margin can then be calculated as:

C(ρ)=SIR−SIR ₀.

-   3. If y=SIR₀+C(ρ) and y=SIR do not intersect for a given SIR₀, then    there is no SIR that satisfies Equation (10) and the system is not    stable under this offered load.

FIG. 5 depicts an example of the determination of C(ρ) using the abovesteps for EDGE mode ECS-5 (210). Points of intersection (510) of theline y=SIR (530) and y=SIR₀+C(ρ) (540) represent stable solutions forthe infinite retransmission model 240. C(ρ) can be calculated by findingthe difference between SIR and SIR₀ at any of these intersection points.For instance, following the steps outlined above, based upon the data inFIG. 5, C(ρ) is approximately −2 dB for SIR₀=6 dB.

Once C(ρ) is calculated according to the above-mentioned steps, it ispossible to calculate the threshold values 260 for a link adaptationsystem based upon an infinite transmission model 240 by simply shiftingthe throughput characteristic curves 250 derived for theno-retransmission model 240 (e.g., FIG. 4). This is evident fromEquation (9) which is of the same form as Equation (1) except for theadditional term C(ρ), the amount by which SIR is reduced due toretransmissions (C(ρ)=SIR−SIR₀). Therefore, the thresholds for theno-retransmission model 240 should be increased approximately by −C(ρ)in order to obtain the thresholds for the infinite transmission model240.

FIG. 6 depicts the derived throughput characteristics for the EDGE modes210 using the infinite retransmission model 240 as outlined herein. Thethreshold values for this infinite retransmission model are obtained byfinding the mode 210 that produces the highest throughput over theentire SIR range. As described earlier, other retransmission models suchas one-retransmission or two-retransmissions can be analyzed using asimilar framework. See J. Chuang, X. Qiu, “An Improved Link AdaptationAlgorithm and Its Implementation Requirements”, presented at SMG2 EDGEad hoc on EDGE physical/link layer issues in London, Aug. 12-13, 1998,and “Link Adaptation in Wireless Data Networks for ThroughputMaximization Under Retransmissions”, AT&T Technical Memorandum,HA6132000-980714-06TM, July 1998, also submitted to IEEE ICC'99, Jun.6-10, 1999.

Analysis of the infinite retransmission model produced a furthercritical discovery that in a retransmission environment there exists acutoff SIR₀, below which there should be no transmissions at atransmitter 100. If a transmitter 100 is operating with SIR below thiscutoff threshold, transmitting will result in system instability, closeto zero throughput and waste of bandwidth resources. For example, anexamination of FIG. 5 reveals that there is no stable solution forEquation (9) if SIR₀ is below approximately 4 dB. This is apparent bynoting that none of the curves y=SIR₀+C(ρ) (540) below SIR₀=4 dB (markedwith ‘x’) intersect the line y=SIR (530). Because SIR is a function ofSIR₀, this means that there is a minimum SIR threshold 260 below whichsystem behavior will become unstable. For example, for the 4 dB value ofSIR₀ from FIG. 5, the corresponding minimum SIR threshold 260 wasdetermined to be approximately 9 dB (see FIG. 6 (610)).

The discovery of this minimum SIR threshold 260 led to a newno-transmission mode (or mode 0) for link adaptation systems. This mode0 (cutoff threshold) is the SIR level at a link receiver 105 below whichtransmission should cease at the corresponding transmitter 100. Iftransmissions continued below the mode 0 threshold 260, systeminstability and near zero throughput would result at the link receiver105. Thus, transmitting below mode 0 wastes bandwidth and systemresources and produces near zero throughput. This no-transmission modeis different from conventional admission control, which is performedonly once upon admitting a user. Mode 0 is part of the continuous linkadaptation process.

FIG. 7 depicts the operation of a no-transmission mode (mode 0)according to one embodiment of the present invention. At time 710, theSIR at the link receiver 105 exceeds the cutoff threshold. Thus, at time710, the corresponding transmitter 100 is transmitting using theappropriate mode X 210 for the current SIR in the link adaptationsystem. At time 720, the SIR at link receiver 105 falls below the cutoffthreshold and the transmitter 100 enters mode 0 ending transmission. Attime 730, however, the SIR at link receiver 105 again exceeds the mode 0cutoff threshold and the transmitter 100 begins transmitting using theappropriate mode Y 210 for the current SIR.

FIG. 8 is a flowchart that depicts a set of steps that may beimplemented at a wireless transmitter to utilize a no-transmission modeand perform link adaptation according to one embodiment of the presentinvention. In step 805, the procedure is initiated. In step 820, asignal quality value is measured at a receiver. The signal quality valuemay be a SIR, BLER or any other value corresponding to the suitabilityof the signal for reception. In step 830, the signal quality value iscompared to a no-transmission threshold value. If the signal qualityvalue is less than the no-transmission threshold (‘yes’ branch of step830), the receiver ceases transmission to the receiver (step 840).Otherwise (‘no’ branch of step 830), link adaptation is performed. Inparticular, in step 850 a best link adaptation mode is selected (e.g., amode that maximizes some performance measure such as throughput). Instep 860, a modulation and/or coding scheme is adjusted at thetransmitter to conform to the best link adaptation mode selected in step850. The procedure ends in step 870.

1. A method for choosing and employing a transmission parameterenvironment between a transmitting unit and a receiving unit whenretransmissions are employed for signals that are not properly received,comprising the steps of: a) determining threshold values that depend onreal-time performance measure for transmissions between saidtransmitting unit and said receiving unit, said determining assumingthat data that failed to be received properly is retransmitted; b) saidreceiving unit measuring said real-time performance measure; c)selecting a transmission parameter that controls data rate based on saidmeasured real-time performance measure and on said threshold values; andd) said transmitting unit adjusting its transmission parameter to saidreceiving unit according to the selected transmission parameter.
 2. Themethod of claim 1 where said real-time performance measure is signal tointerference ratio (SIR) values.
 3. The method of claim 1 where saidreal-time performance measure is bloc error rate values.
 4. The methodof claim 1 where said transmission parameter specifies a modulationmode.
 5. The method of claim 1 where said transmission parameterspecifies a coding scheme.
 6. The method of claim 1 where saidtransmission parameter specifies both a modulation mode and a codingscheme.
 7. The method of claim 2 where said thresholds are such that atan SIR value below a preselected threshold, said step of selecting atransmission parameter chooses a parameter that corresponds to notransmission at all.
 8. A method for performing wireless link adaptationin a retransmission environment between a transmitting unit and areceiving unit comprising the steps of: said receiving unit measuringquality of signals received from said transmitting unit; if said qualityis below a preselected threshold, sending a message to ceasetransmitting to the receiver; if said quality is not below saidpreselected threshold, selecting a link adaptation mode, and adjustingmodulation, or coding scheme, or both to correspond to the selected linkadaptation mode.